1. Field of the Invention
The present invention relates to a transmission liquid crystal display and a method of forming the same, and more particularly to a transmission liquid crystal display with an improved contact hole structure and a method of forming such contact holes therein.
2. Description of the Related Art
In recent years, liquid crystal displays have widely been used in various fields due to their advantageous light weight and low power consumption. A transmission active matrix liquid crystal display is a highly attractive liquid crystal display. Such a display has an active matrix substrate and an opposite substrate, which together define an inter-space filled with liquid crystal. The active matrix substrate has plural thin film transistors, which serve as switching devices for switching display pixels, wherein the display pixels are provided over the active matrix substrate. The opposite substrate is transparent, for allowing transmission of light. It is important for such a display to have improved contrast and color and also increased display screen area.
In order to achieve such improvements, it is necessary to increase transmittivity of a back-light emitted from a back-illuminator, which illuminates a liquid crystal panel.
Advanced liquid crystal displays reduced weight, thickness and power consumption. In order to reduce power comsumption, it is quite effective to save or reduce a power comsumption by the back-illuminator.
Regarding improved transmittivity of the back-light, it is effective to increase an aperture efficiency of a display portion, which includes pixels. In view of increasing the aperture efficiency, the display has the following structural elements.
The liquid crystal device has a planarized transparent organic insulating film, which covers entire regions including the interconnections coupled with the electrodes and the thin film transistors, wherein an aperture comprises a transparent film which permits light-transmission. An inter-layer insulator as a protector is further provided, which overlies the substrate and underlies the planarized transparent organic insulating film. Transparent pixel electrodes are provided over such a planarized transparent film. Contact holes are formed, which penetrate laminations of the planarized transparent organic insulating film and the inter-layer insulator, so that contact plugs are then formed in the contact holes, whereby the transparent pixel electrodes over the planarized transparent film are electrically connected through the contact plugs to interconnections, which underlie the inter-layer insulator, wherein the interconnections are coupled with the electrodes of the thin film transistors over the substrate.
The above inter-layer insulator comprises an inorganic insulating film, whilst the planarized transparent film comprises an organic insulating film. This means that the contact holes penetrate the laminations of the organic and inorganic films. The contact holes are formed by an etching process for selectively etching such laminations, before the contact holes are filled with the contact plugs by a sputtering process. The etching process and the subsequent sputtering process, however, damage the planarized transparent organic insulating film, whereby the film is deteriorated in light-transmittivity. This means that the light-transmittivity of the apertures is reduced.
Japanese laid-open patent publication No. 10-20342 discloses contact holes which penetrate laminations of an overlying photo-sensitive organic insulating layer and an underlying inorganic insulating layer. A photo-lithography technique may be utilized to form the contact holes in the overlying photo-sensitive organic insulating layer, before a dry etching process is carried out with use of the processed overlying layer as a mask for forming the contact holes. The dry etching process uses an etching gas which contains carbon, fluorine and hydrogen in order to prevent deterioration of the overlying photo-sensitive organic insulating layer and also prevent the underlying layer from side-etch.
Japanese laid-open patent publication No. 11-283934 discloses another conventional technique to prevent any substantial increase in a resistance value of the contact plug in the contact hole. Contact holes are formed in a transparent resin inter-layer insulator by an etching process, by which residues remain on bottoms of the contact holes. A further sputter cleaning process is carried out to remove residues from the bottoms of the contact holes because the residues may increase the resistance, wherein the residues are removed by impacts of sputtered particles in hydrogen, helium or nitrogen atmosphere.
FIG. 1A is a fragmentary cross sectional elevation view of such a sputter cleaning process for removing the residues from the contact holes. The planarized top surface of the transparent organic film 8 is directly exposed to the sputtered particles, whereby the planarized surface becomes a rough surface 17a, and an upper region 17b of the transparent organic film 8 is damaged and deteriorated in film quality. FIG. 1B is a fragmentary cross sectional elevation view of the transparent organic film with the damaged upper region by the sputter cleaning process of FIG. 1A. The above organic film 8 has a C—C bonding structure. In the upper region 17b of the organic film 8, this C—C bonding structure may be broken by the sputtered particles, whereby the molecular structure of the upper region 17b is changed, for example, side chains of the molecule structure are broken. As a result, the above rough surface 17a is formed. This rough surface 17a causes a remarkable decrease in light-transmittivity of the transparent organic film 8.
In the above circumstances, it would be advantageous to develop a novel transmission liquid crystal display and method of forming the same free from the above problems.